The present invention relates to a wave soldering method and to an electronic board product that has at least one component on the topside of the board soldered with solder joints which are prone to topside reflow during wave soldering of the board. The invention is particularly useful for boards which have dense via structures such as via-in-pad and via-between-pad design structures.
The secondary reflow of solder joints of topside, surface mounted components on mother boards and server boards during the wave soldering process of the boards is a known problem. Three causes of this topside reflow of surface mounted components are shown in FIG. 1. The primary cause is the conduction of heat, represented by arrows A in FIG. 1, from the solder wave 13 against the lower side of the board 14 through vias 15 in the board, by way of the via hole plating 16 forming the through vias, to the surface mount land 17 and solder joint(s) thereon on the topside of the circuit board. A secondary reason is the radiation of heat, see arrows B in FIG. 1, from heaters 18 above the board, which are primarily there to apply pre-heat. A third reason is the transfer of heat, depicted by arrows C in FIG. 1, by conduction through the laminate material of the board 14. Recent development of via-in-pad design structures increases the likelihood of topside reflow of ball grid array (BGA) component, 19 in FIG. 1, solder joints whose land patterns have these via-in-pad structures designed in.
Several known techniques for alleviating the problem of topside component reflow are shown in FIG. 2. A non-metallic solder wave shield 20 is placed under the bottom of the board, to prevent the solder from touching the vias. This wave shield is typically attached to the wave solder pallet. A heat shield 21 can also be placed above the board on the topside, covering all components prone to topside reflow. The holes 22 of the vias 15 can also be capped as shown at 23 with solder mask material to prevent the solder wave from getting into the via holes. An example of a shield and method for selective wave soldering are disclosed in U.S. Pat. No. 5,704,535. See also U.S. Pat. No. 5,739,586 which discloses the use of tape over the lower end of via holes to hold solder in the holes. U.S. Pat. No. 5,888,627 discloses filling through holes for vias with an electroconductive resin paste. Typically, materials used to fill via holes in printed circuit boards are epoxy based thermoset materials which do not undergo a phase change during the wave soldering process.
Vias are to be distinguished from through holes for component leads in electronic boards. Vias are smaller in diameter than component lead through holes, 10 to 13 mils diameter holes are used for vias, for example, as compared with 22 to 36 mils diameter holes for component lead through holes. Also, the primary function of a component lead through hole is to form a solder fillet with a component lead protruded into the through hole, whereas the primary function for a via is electrically connecting traces on different layers of the board. There is no component lead in a via but both via holes and component lead through holes have copper barrel plating therein. Vias are designed as small as possible to take up minimal space on the board. Solder will always fill a component lead through hole during wave soldering, but not necessarily so for a via.
Temporarily plugging through holes for component leads during wave soldering for preserving the holes in an unobstructed manner is taught by U.S. Pat. No. 4,373,655. A pin of a plastic material, e.g., glass filled nylon, that is stable in the presence of the high temperatures commonly encountered in mass soldering processes is placed in the through hole during soldering and removed after the soldering operation. The patent also refers to the use of latex tape placed over the ends of the holes to be spared on the underneath side of the printed circuit board, to mask such holes from solder. The use of wooden toothpicks and uniformly tapered, thermally conductive metal pins are also disclosed for this purpose.
The use of wax as a cement or jig to hold a component lead in position in a through hole in a circuit board during wave soldering to through hole mount a component on the board is known, see U.S. Pat. Nos. 4,180,199, 4,127,629, 3,973,322, and 3,966,110, for example. In these processes, the wax melts upon exposure to the molten solder during wave soldering and is removed from the board and component leads.
U.S. Pat. No. 6,202,739B1 is cited of interest for its disclosure of an apparatus, particularly a hand-held electronic device such as a cellular telephone or two-way radio, including a heat dissipating apparatus for effectively removing heat from a heat-generating electronic component of the apparatus, and a method of forming the same. An endothermic, phase change material is enclosed within a sealed cavity in the apparatus where it is thermally coupled to the heat-generating component therein to dissipate heat therefrom during operation of the electronic device.